JPH09150203A - Manufacture of low carbon steel sheet excellent in surface property - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a low carbon steel sheet without ridging or the like and excellent in surface property. SOLUTION: This method is a method specified in hot rolling conditions of the low carbon steel sheet, especially, in hot finish rolling conditions and its essential points are that, in the manufacturing method of the low carbon steel sheet having C content of <=0.1wt.%, at the time of hot-rolling this low carbon steel, the surface roughness of rolling rolls used for finish rolling is taken as >=1.5μm. Further, at the time of rolling the steel with the rolling rolls having this roughness, at least one or more passes of different circumferential speed rolling of different circumferential speed rate of >=15% are executed or thickness at the time of the completion of hot finish rolling is taken as <=2mm.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a low carbon thin steel sheet having good surface characteristics. Here, the surface characteristics in the present invention refer to surface properties that deteriorate when the hot rolling temperature is low and is below the Ar ₃ transformation point and α phase is precipitated. Specifically, it means ridging (also called roping) that occurs when hot rolling is performed in the α region.

[0002]

2. Description of the Related Art Conventionally, in hot rolling of a low carbon steel sheet, it is necessary to set the end temperature thereof to an Ar ₃ transformation point or higher (hereinafter referred to as γ-region hot rolling) so that the surface property of a cold rolled / annealed sheet is deteriorated (particularly It has been essential to prevent ridging). However, in recent years, hot rolling below the Ar ₃ transformation point (hereinafter referred to as α-area hot rolling) has been newly researched and developed in recent years from the viewpoint of cost reduction (low-temperature slab heating and cold rolling steps are simply omitted). For example, in Japanese Patent Laid-Open No. 61-119621, α
It is disclosed that deep drawability is improved by hot rolling in the zone.
However, when such hot rolling in the α region was carried out, ridging, which did not occur in the conventional hot rolling in the γ region, became apparent and could not be put to practical use.

As techniques for improving ridging due to the hot rolling in the α region, there are 18 publications from JP-A-61-204320 to JP-A-61-204338, and JP-A-61-261434, 261435. There is a bulletin, etc. All of these publications disclose a technique for improving the ridging of a product sheet manufactured by omitting the cold rolling step by increasing the rolling reduction and strain rate during hot rolling in the α region. However, during hot rolling, especially the high pressure reduction rate or high strain rate in the latter half of the final hot rolling not only causes operational problems such as stripability but also promotes the occurrence of hot rolling defects and lowers the yield of product sheets. It also causes.

Further, in these prior arts, there is no clear description about the ridging property of a thin steel sheet produced by cold rolling after hot rolling in the α region. It is generally known that the ridging characteristics deteriorate with cold rolling reduction, and it is considered that the ridging characteristics of the cold rolled / annealed sheet cannot be improved by the ridging characteristic improving effects of these prior arts. However, in the steel sheets for automobiles and beverage cans, which are the main applications of low carbon thin steel sheets, the cold rolling step is essential from the viewpoint of surface beauty, and the ridging property is also exhibited when the cold rolling step is performed after the α area hot rolling. The development of a technology that does not deteriorate the quality of water is an issue.

[0005]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention The present invention is realized when a manufacturing method for improving the surface characteristics of a low carbon thin steel sheet while overcoming the above problems of the prior art, particularly when hot rolling of the α region is carried out. It is an object of the present invention to provide a manufacturing method capable of improving the ridging characteristics of

[0006]

In order to achieve the above object, the present invention specifies the hot rolling conditions of a low carbon thin steel sheet, especially the finish hot rolling condition.
In the method for producing a low-carbon steel sheet having a content of 0.1% by weight or less, in the hot rolling of the low-carbon steel, a rolling roll used for finish rolling at a hot rolling temperature of Ar ₃ transformation point or less is used. The surface roughness is to be 1.5 μm or more.
Further, when rolling with a rolling roll having the roughness, at least one pass or more of different peripheral speed rolling with a different peripheral speed ratio of 15% or more, or the finish hot rolling finish plate thickness of 2 mm or less, the present invention The effect of is even more exerted.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
It is generally considered that ridging is manifested by the plastic anisotropy of crystal colonies present in the steel sheet. A crystal colony is a group of adjacent crystal grains whose crystal orientations are substantially the same, and the most prominent colony as a cause of ridging is a {100} colony whose {100} orientation is parallel to the plate surface normal direction. . With respect to this {100} colony, research is progressing on an α-type stainless steel sheet in which ridging occurs remarkably, and it is considered that the colony should be destroyed to improve ridging. In the case of low carbon steel, in normal γ-region hot rolling, after hot rolling, it is completely transformed into α phase, fine grains and randomization occur, and colonies are not formed.

On the other hand, when hot rolling in the α region is performed, colonies are formed in the α phase during hot rolling, colony formation is promoted during the subsequent cold rolling, and colonies remain even after annealing, thus improving the ridging property. It is considered to deteriorate. Therefore, in order to improve the ridging property, it is sufficient to inhibit the colony generation during hot rolling in the α region. Therefore, the inventors of the present invention have devised and completed the present invention by carefully studying the relationship between the hot rolling conditions and the ridging characteristics in the cold rolling / annealing plate regarding the colony destruction during the α hot rolling.

The reasons for limiting the method of the present invention will be described below. First, the content of C is limited to 0.1% or less because the workability as a low carbon thin steel sheet is not satisfied with the content of more than 0.1% and the production cost is increased by the addition of C.

The reason why the hot rolling temperature is limited to the Ar ₃ transformation point or lower is that the steel is completely in the γ phase in the temperature range higher than that, and ridging hardly occurs when the hot rolling is completed at the temperature. Because.

The reason why the rolling roll roughness during hot rolling below the Ar ₃ transformation point is limited to 1.5 μm or more is that the ridging property of the product sheet is not improved with the roll roughness below this. The upper limit thereof is not particularly specified, but if the roll roughness is too large, the surface quality of the steel sheet is deteriorated, and it is usually up to about 50 μm. The roll roughness in the present invention is indicated by the average roughness Ra defined by JIS. Here, the lower limit of the rolling temperature by the roughness roll is not particularly specified, but it is usually up to 300 ° C., and if it is lower than that, the occurrence of hot rolling is remarkable.

Further, although the total reduction ratio after the steel sheet temperature becomes Ar ₃ or lower (that is, hot rolling in the α region) is not particularly specified, if the total reduction ratio is 10% or less, the ridging becomes remarkable. However, from the point of the present invention, the higher the total rolling reduction, the greater the effect of improving the ridging characteristics. Although the rolling reduction per pass is not particularly specified, it is usually about 10-60%, and if it is less than that, it impairs the stripability, and if it is more than that, not only hot rolling defects but also roll wear becomes remarkable, resulting in cost increase. Bring Further, the influence of the strain rate during rolling on the function and effect of the present invention is extremely small, so it is not necessary to specify it, but an excessively high strain rate (for example, 300 s-1) impairs the stripability.

The roll roughness in the present invention may be basically the roll roughness before rolling. However, in a hot rolling mill used for industrial production, a large amount of steel sheets are continuously rolled, so that the roll may be worn and the roll roughness may be lowered during rolling. At this time, it is desirable from the viewpoint of productivity to polish the rolls without interruption of rolling by the recently developed online roll polishing machine to maintain the roughness of the present invention.

Next, the reason why the invention is limited to claim 2 will be described. The reason why the different peripheral speed ratio is set to 15% or more when carrying out the hot rolling according to claim 1 is that the ridging characteristic is not improved at a different peripheral speed ratio lower than that of the invention method in the case where the different peripheral speed rolling is not carried out. . The number of rolling passes is sufficient if it is 1 pass. In the present invention, the upper limits of the different peripheral speed rate and the number of different peripheral speed rolling passes are not particularly defined, but it goes without saying that the larger the difference, the greater the effect of improving ridging. But,
Different peripheral speed ratios of 50% or more are currently difficult, and the number of finishing hot rolling passes is usually up to about 8. Here, the different peripheral speed ratio in the present invention is a value obtained by dividing the peripheral speed difference between the upper and lower rolling rolls by the peripheral speed of the low peripheral speed side roll, and is expressed as a percentage. Further, in the different peripheral speed rolling of the present invention, there is no difference in the ridging improvement effect regardless of which peripheral roll speed is higher.

In the third aspect, the reason why the finish hot rolling finish plate thickness is limited to 2 mm or less is that the ridging property is not improved more than the present invention with a plate thickness larger than this. Also,
It goes without saying that the thinner the plate thickness, the greater the ratio of the strain distribution changing portion to the total plate thickness, and the greater the effect of improving ridging. However, the lower limit of the finished sheet thickness by ordinary hot rolling is about 1 mm, and even the continuous hot rolling mill which has been developed in recent years reaches up to about 0.5 mm thickness.

By the way, the reason why the ridging property of the product sheet is remarkably improved by increasing the roughness of the hot-rolled roll, or by reducing the peripheral speed rolling or the plate thickness after rolling is not always clear at present. , Can be considered as follows. As described above, the destruction of {100} colonies improves the ridging property of the product plate. According to the studies by the present inventors, it is considered that the formation of colonies occurs as follows. First, when the crystal is rotated during rolling deformation (plane strain deformation) and the {100} orientation becomes parallel to the plate surface normal direction, the orientation of each crystal grain remains the same even if it is recrystallized and refined in the subsequent annealing step. Even after recrystallization, the orientation is close to {100}, and a group of these crystal grains becomes {100} colonies. Therefore, if the formation of {100} orientation at the time of plane strain deformation is prevented or the colony size is reduced by making the grains finer (further randomization) before the plane strain deformation, the ridging characteristics are improved. For example, if the conventional hot rolling of the γ region where α ⇔ γ complete transformation occurs and α grains are refined and randomized before cold rolling (plane strain deformation), ridging does not occur in the cold rolled and annealed sheet. . Also, in ferritic stainless steels where ridging has become more obvious than before,
Since the complete transformation does not occur, the hot rolling process is also considered to be the {100} forming plane strain deformation process. Furthermore, in low carbon steel,
Considering that ridging may occur when hot rolling in the α region is performed, the hot rolling process in the α region of low carbon steel can be regarded as the initial colony forming process.

Assuming that the above temporary arrangement is correct, the effects of roll roughness, different peripheral speed hot rolling, and finish hot rolling in the present invention are as follows.
It can be considered that the ridging property was improved by inhibiting the colony formation in the α region hot rolling process and reducing the colonies on the product plate. That is, it is considered that when the roll roughness is increased, the friction coefficient between the steel plate and the roll is increased, shear strain deformation occurs in the steel plate surface layer, the strain distribution is changed, and the planar strain condition is not maintained and colony formation is inhibited. . Further, it can be considered that the different peripheral speed hot rolling promotes this shear deformation, and the thinning of the plate thickness increases the ratio of the strain distribution changing portion to the total plate thickness. However, it is generally considered that the colony existing in the center of the plate thickness causes ridging, and it is not clear at present how the change in strain distribution of the surface layer of the steel plate acts on the colony formation in the center of the plate thickness. Further, since the roll roughness effect is recognized even when performing hot rolling by lubrication, the effect is not attributed only to the friction coefficient between the steel plate and the roll as described above, and the steel plate is caused by the geometric unevenness of the roll surface layer. It is considered that the effect of strain change in the surface layer is also large. Furthermore, if the surface layer shear deformation has a ridging-improving effect, the ridging characteristics should be improved simply by carrying out different peripheral speed hot rolling. There is no. That is, it is considered that the ridging improvement effect is inherent in the change in the steel sheet surface layer strain distribution when the roll roughness is increased.

[0018]

EXAMPLES The present invention will be described in detail below with reference to examples. (Example 1) After heating low carbon steel plates (steel types A and B) having a chemical composition shown in Table 1 and having a thickness of 25 mm to 1000 ° C, steel type A was used.
Was air-cooled to 900 ° C. and steel type B was cooled to 850 ° C., hot rolling was performed for 6 passes, and a hot rolling treatment was performed at 550 ° C. for 1 hour after hot rolling. Details of the hot rolling conditions at this time are shown in Table 2,
It is shown in Table 3. The hot-rolled sheet obtained was pickled and then subjected to a total rolling reduction of 80.
% Cold-rolled and annealed at 800 ° C. for 30 seconds, and then evaluated for ridging characteristics. The results are shown in Tables 2 and 3 and FIG.
The results are shown in Figs.

The ridging characteristics were evaluated as follows. Tensile test pieces (10 pieces) parallel to the rolling direction from the product sheet
Was cut out and the maximum value of roughness (each test piece) when tensile strain was applied to each test piece in the rolling direction was obtained, and the average value (for 10 pieces) of the maximum value was taken as the ridging height. With this evaluation method,
If the ridging height is about 20 μm or less, the ridging property can be said to be good, and it will be 10 μm or less when manufactured by the conventional γ-region hot rolling.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

[Table 3]

FIG. 1 is a view showing the influence of the hot rolling roll roughness on the ridging characteristics when the thickness of the hot rolled finished sheet is constant.
The results for steel types A and B are shown. In the figure, the circle indicates steel type A
The triangle mark indicates the result of the steel type B, and the result of the symbol A-1 in Table 2 and the result of B-1 in Table 3 of the comparative method are indicated by the white mark.
The result (A-6, 7,
14 and B-6, 7, 14) are represented by black marks. From the figure,
It is recognized that when the roll roughness during hot rolling is set to 1.5 μm or more, the ridging height of the cold rolled annealed sheet becomes 20 μm or less, and the ridging characteristics are improved.

FIG. 2 shows the hot rolling roll roughness of 1 μm and 3 μm.
2 is a diagram showing the effect of different peripheral speed rolling ratios on the ridging characteristics when the two levels of A, B-1, 2,
3, 4 (comparative method: white circles and white triangles in the figure) and symbols A, B-
7, 8 (the first method of the present invention: black circles and black triangles in the figure), and further reference signs A, B-9, 10, 11 (the second method of the present invention (claim 2): a symbol with diagonal lines in the figure) ) Shows the result. As is clear from the figure, the ridging height is reduced when the roll roughness is large, and the ridging characteristics are further improved and the ridging height is increased when the different peripheral speed rolling ratio is 15% or more when the roll roughness is large. It is recognized that it becomes 10 μm or less.

FIG. 3 is a diagram showing the ridging characteristics when the hot-rolling finish plate thickness is changed by setting the hot-rolling roll roughness to two levels of 1 μm and 3 mm. 5 (comparative method: white circles and white triangles in the figure), symbols A and B-7 (first method of the present invention: black circles and black triangles in the figure), and symbols A and B-12,
13 (invention method claim 3 (claim 3): a symbol with diagonal lines in the figure). From the figure, the ridging property becomes good when the roll roughness is large, and the ridging property of the cold rolled annealed plate is further improved by reducing the thickness of the hot-rolled finished plate when the roll roughness is large, and the ridging height is 10 μm. It is recognized that

Example 2 A low carbon steel having the chemical composition shown in Table 4 was melted according to a common melting method to obtain a 250 mm thick continuously cast slab. After heating the slab, hot rolling,
Subsequently, a hot rolling coil was formed by a 7-pass finishing hot rolling mill. The hot rolled coil thus obtained is subjected to pickling, cold rolling and annealing to 0.2
The product plate has a thickness of mm. The ridging characteristics of these product plates are
Table 5 shows the manufacturing conditions. In addition, a part of the hot rolled coil
In order to evaluate the ridging property of the hot rolled sheet, a tensile test piece was cut out after pickling, a tensile strain was applied in the rolling direction, and the ridging property was visually evaluated. The ridging characteristics of the cold rolled annealed sheet were evaluated in the same manner as in Example 1.

[0027]

[Table 4]

[0028]

[Table 5]

Table 5 shows the result of hot rolling in the γ region in accordance with the conventional method. The ridging property is extremely good, but high temperature slab heating is required to set the finish hot rolling end temperature to the Ar ₃ transformation point or higher. Become. Particularly, in order to reduce the load in the cold rolling process, when the thickness of the hot-rolled finished plate is reduced, further high temperature slab heating is desirable. In the case where the hot-rolling finish plate thickness is thinned at the same heating temperature as, the hot-rolling finish temperature is lowered to become the α-region hot rolling, and ridging is significantly exhibited on the product plate,
It cannot be put to practical use. Shows the case where the slab heating temperature was lowered and the same hot-rolling finish sheet thickness was used. Even if the thickness of the hot rolled finished sheet is the same as that of the conventional method, since the heating temperature is low, it becomes hot rolling in the α region, and ridging appears similarly to the above.

In contrast to the above-mentioned conventional method or comparative method,
The cold-rolled and annealed sheet of the present invention subjected to the method of the present invention showed a good ridging characteristic of 20 μm or less in spite of the α-region hot rolling. In particular, the materials using the second and third methods of the present invention also show the characteristics of conventional γ-region hot rolled materials. Further, from the viewpoint of omitting the cold rolling step, it is clear that the ridging characteristics of the material according to the method of the present invention are good even when comparing the ridging characteristics of the material having a thin hot rolled sheet thickness and the material of the hot rolled sheet.

[0031]

As described in detail above, according to the present invention, the ridging characteristics during hot rolling of the α region, which has been inevitable in the past, can be improved without causing operational problems such as stripability during hot rolling. Therefore, it is possible to reduce the manufacturing cost due to the low temperature slab heating and the omission of the cold rolling process, which is a great industrial benefit.

[Brief description of the drawings]

[Fig. 1] Low-carbon steel with a thickness of 25 mm is changed to 3 mm by changing the roll roughness.
After performing hot rolling to a thickness and performing a winding-equivalent treatment at 550 ° C. for 1 hour, it was pickled and then cold rolled at a total reduction of 80% to obtain 8
The relationship between the ridging property of the sheet annealed at 00 ° C. for 30 seconds and the hot rolling roll roughness is shown. In the figure, the white mark means the production by the comparative method, and the black mark means the production by the method of the present invention. Further, the circles in the figure mean the results for steel type A in Table 1, and the triangles mean the results for steel type B in Table 1. The roll roughness was evaluated by the average roughness Ra defined by JIS.

[Fig. 2] 25 mm thick low carbon steel is hot rolled to 3 mm thick,
Then, the ridging characteristics of the sheet cold rolled to 0.6 mm and annealed at 800 ° C. for 30 seconds were shown with respect to different peripheral speed ratios during hot rolling. Roll roughness during hot rolling is 1 μm (white mark in the figure) and 3 μm
(Black mark in the figure) was set to two levels, and the peripheral speeds of the upper roll and the lower roll were changed to carry out different peripheral speed hot rolling. The white mark in the figure means that it was manufactured by the comparative method, and the black mark and the shaded black mark in the figure were respectively manufactured by the first method (claim 1) and the second method (claim 2) of the method of the present invention. Means that. The circles and triangles mean the results of steel types A and B shown in Table 1, respectively.

FIG. 3 shows ridging characteristics when the thickness of the hot-rolled finished sheet is changed in the same experiment as in FIG. At this time, the hot rolling roll roughness was set to two levels of 1 μm (white mark in the figure) and 3 μm (black mark in the figure). The white marks in the figure mean that they were manufactured by the comparative method, and the black marks and the black marks with diagonal lines in the figures were respectively manufactured by the first method (claim 1) and the third method (claim 3) of the method of the present invention. Means that. The circles and triangles mean the results of steel types A and B shown in Table 1, respectively.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takashi Oda 1-1, Toibata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka Shin Nippon Steel Co., Ltd. Yawata Works

Claims (3)

[Claims] 1. A method for producing a low carbon thin steel sheet having a C content of 0.1% by weight or less, wherein when hot rolling the steel, the hot rolling temperature is below the Ar ₃ transformation point and the surface roughness is 1. ．
A method for producing a low carbon thin steel sheet having good surface properties, which comprises rolling using a roll having a size of 5 μm or more. 2. The hot rolling according to claim 1, wherein at least one different peripheral speed rolling having a different peripheral speed ratio of 15% or more is performed.
A method for producing a low carbon thin steel sheet having good surface characteristics, characterized by applying more than one pass. 3. A method for producing a low carbon thin steel sheet having good surface characteristics, wherein the hot rolling finish finish is 2 mm or less in the hot rolling according to claim 1.